The invention concerns white pigments used in paints, lacquers and primers, and for their use in an improved method for the creating a white “pearl” finish on a painted or lacquered surface.
In the automotive industry a pearlescent white finish is a desirable and highly prized finish colour for automobiles and trucks. A pearlescent white finish has been found to be more popular with purchasers than a flat white finish.
Conventionally, in order to create a pearlescent white finish on an automobile, a three step application process is necessary after the exposed metal parts have has been primed with one or more coats of primer. The following steps are employed to create the pearlescent white finish: a base coat of white paint is applied, next a coating of pearlescent material usually containing mica, but which is not pigmented, but rather contains particles of a highly reflective material to provide the “pearl” finish. Finally, a clear coat is applied to seal the underlying layers. Each coating layer must be applied separately, and suitable drying times, and if necessary baking, must be provided prior to the application of each subsequent layer. Consequently the production of a pearlescent finish is labour intensive and time consuming. Whenever coats of two different types of paint are applied to an article, it is necessary to leave a longer drying time than is needed between multiple coats of the same type of paint. In order to facilitate the proper setting of a three coat paint finish, drying times are required to set each of the paint undercoat, the pearl coat and the clear coat. The longer drying times are achieved by having the painted articles on the assembly line for additional periods of time. It is not feasible to simply run the assembly line more slowly through the painting section of an automobile assembly plant in order to create longer drying times between paint applications. Instead, the entire assembly line must be lengthened to provide the further travel time during which paint drying can occur. It follows that in order to make the assembly line longer, a larger physical plant space is needed through which to run the assembly line. In manufacturing plants where physical space is at a premium, it may not be possible to set aside additional assembly line length to facilitate multiple times between the application of multiple paint coats. Accordingly, it may be impossible to offer certain paint finishes on vehicles manufactured at some smaller manufacturing plants.
U.S. Pat. No. 5,871,827 to Jaffe et al. (the entire subject matter of which is incorporated in by reference) discloses two coat and three-coat automotive finishes and a process for their preparation wherein polychromism is achieved in incorporating an opaque light interference pigment into to the top coat or the mid coat. The processes taught by Jaffe involve the use of two coating layers, the first of which contains an opaque white, black or coloured pigment to achieve total hiding and the second layer being an interference pigment so that it does not hide the first coating. A clear topcoat is still applied over the pigmented coats in order to compete the finish. Moreover, the particles of opaque interference pigment taught are multi-layered particles having an opaque layer coated with a transparent dielectric layer, in turn coated with a semitransparent layer. This would be a relatively complex and expensive coating process to apply, and the result of the process are polychromatic paint effects.
U.S. Pat. No. 5,350,509 to Sada et al. (the entire subject matter of which is incorporated in by reference) discloses a coating structure that includes a colour base, a first coating layer placed on the colour base, and transparent brilliant members contained in the first coating layer, each brilliant member including an alternate lamination of at least two polymers having different refractive indexes and controlling interference light resulting from reflection interference produced by alternate lamination and transmitted light other than interference light.
U.S. Pat. No. 4,499,143 to Panush (the entire subject matter of which is incorporated in by reference) discloses a transparent topcoat composition containing low pigment to binder ratio of iron oxide encapsulated in mica particles. The teachings of Panush are particularly addressed to producing coloured finishes having depth, clarity and chromaticity. Although this patent states that mica encapsulated iron oxide pigments have inherent hiding capabilities in addition to being pearlescent, they are said to be additive colours. These desired colour results are stated not to be obtainable with metals (i.e aluminum) and previous pearlescent pigments (natural or synthetic). These so called “colorless and opaque pigments reduce the value of the true coloured pigments and resulting in gray-cloudy-low chroma colours. In view of the focus on coloured finishes, this patent fails to recognize and teach the value of aluminum as a pigment and hiding material in pearlescent white finishes.
U.S. Pat. No. 4,615,940 to Panush (the entire subject matter of which is incorporated in by reference) discloses an opalescent colour effect on a substrate utilizing a multicoat system in which a coloured primer is applied to a surface, followed by a transparent basecoat, and then a clearcoat. This transparent basecoat should tend to have poor hiding qualities, and relatively high light transmittance. In order to compensate for this feature of the basecoat, primers with very effective black and white hiding would be required, and further measures would be necessary to block UV light levels, otherwise there could be significant electrocoat degradation through the transparent basecoat. Additionally, the use of a transparent basecoat requires that the primer layer must be applied carefully and sanded thoroughly, since the transparent basecoat would fail to mask defects in the primer coat.
In the case of conventional titanium dioxide white paint, very large amounts of titanium dioxide pigment must be added to the paint medium in order to achieve adequate hiding power at a reasonable film thickness, typically 30 to 50 microns for conventional painting applications. The paint formulation has very little ability to hold more solid particulates, so there is little room in the paint mixture left to add a pearlizing material such as particles of mica. It is already known that adding mica to a conventional white titanium dioxide paint does not change the visible appearance of the white paint. Conventional titanium dioxide-based white paint, that is paint having a titanium content exceeding 90 percent, of the pigment portion, continues to have a “flat” white appearance despite the addition of a pearlizing material.
Solvent base paints can carry from 40 to 60 percent pigment. Water based paints and primers, on the other hand, do not atomize the pigment as well. Accordingly, the maximum of pigment loading should be much lower, for example in the range of 20 to 35 percent. In each case, for conventional white paint, 90 percent of the pigment must be white pigment, namely titanium dioxide.
It is believed that the mica flakes are drowned in the large quantity of titanium dioxide pigment which is needed to create conventional white paint, and the pearlized appearance of the mica is lost to the eye.
In order to create a conventional painted finish which has a white pearlescent appearance, it has been necessary to apply a pearl coating of mica or another pearlizing material after the white titanium dioxide paint has set in place in order to prevent the “drowning” of the pearlizing material. A conventional pearl coat has no hiding power of its own. The pigmented base coat is required for the hiding properties of the finish, so as to cover sand marks etc. on the article and to create the actual colour of the painted article. The pearl coat merely gives the shiny characteristic that converts a “flat” finish into a “pearl” finish. The titanium dioxide white base-coat and the pearl coat are two different types of paints which require distinct flash times in order to set the coats before other substances are applied to the surface. If no flash time is allowed between the application of a pigmented base coat and a pearl coat paint striking will occur between the coats. Paint striking is a painting flaw which occurs when the dyes or pigments from a previous paint layer become dissolved in the solvents of a newly applied material and seep through to alter the finish colour of the newly applied material. Striking is caused by the application of two different types of paint coatings without a long enough flash time between the application of the two coatings. The application of the separate mica pearlizing coating necessitates a separate application step which requires significant adaptation of automotive production lines and causes a substantial increase in the painting cost of each vehicle produced. The effect of exposure to sunlight is the fundamental cause of the weathering deterioration of most materials. The primary component of paint weathering is photo degradation.
The wavelength distribution of sunlight that reaches the Earth's surface is important because of the relative effect on the material caused by each wavelength region.
Sunlight can be divided into three major regions: ultraviolet (UV), visible, and infrared (IR). Each region has its own distinct wavelength range.    1. UV wavelengths less than 400 nm 6.1% composition of the sunlight.    2. Visible wavelengths between 400–700 nm—51.8% composition of sunlight.    3. Infrared wavelengths above 700 nm—42.1% composition of the sunlight.
The visible region contributes to the largest portion of the overall solar energy; however, it is the UV portion of the sun's energy that is the most destructive element. The ultraviolet by itself can be divided into three distinct wavelength ranges; only the UVA and the UVB reach the Earth's surface.
RANGEWAVELENGTHS (nm)UVCLess than 280UVB280–320UVA320–400
There is no UVC at Earth's surface as wavelengths below 293 to 300 nm are filtered out by the atmosphere. The shorter the UV wavelength, the more damaging its effects on materials. The UVA and the shorter wavelength UVB are responsible for most photo degradation. Therefore the range of sunlight that comprises the smallest percentage of the solar spectrum is the primary cause for material degradation
Photodegradation occurs as a result of light energy breaking a chemical bond in the exposed material, causing a deterioration of the physical structure. As the wavelength becomes shorter, the energy of each individual packet becomes greater, allowing the photon to break progressively stronger molecular bonds. Thus chemical structures able to withstand irradiation at 350 nm may not be able to endure radiation at 320 nm. As the energy level in the photons increases, however, there is a reduction in quantity available.
The first law of photochemistry states that only light that is absorbed can cause damage. Thus, if the absorbance of the damaging UV energy can be prevented or otherwise reduced, deterioration will be slowed. This premise forms the basis for much of the research in paint formulation.
The visible portion of the solar spectrum is responsible for a limited amount of physical degradation and only in a few materials that are susceptible. Some dyes and pigments are sensitive to wavelengths in the lower regions of the visible spectrum. This manifest a colour changes in most materials but without changes to other physical properties.
The infrared region causes heat buildup to occur on radiated specimens, but has not otherwise been associated with causing significant deterioration to occur. The IR is a factor in the deterioration because absorption of these wavelengths cause specimen temperature to rise, which in turn leads to an increase in the rate of photo degradation.
While important strides have been made over the years to improve UV protection for coatings, there remains a need for an improved pearlescent coating with UV filtering capabilities.
It is therefore an object of the present invention to address at least some of the above mentioned disadvantages.